Permanently joined plate heat exchangers are known for instance by GB 0580368 and GB 2126703. These may be produced in shape of all-welded plate heat exchangers in such a way that the heat transfer plates first are welded together in pairs along an inner line and then two such pairs of plates are welded together along an outer line. An all-welded plate heat exchanger may also be produced in that several heat transfer plates are welded together simultaneously, however, the size of the plate heat exchanger becomes limited to the number of heat transfer plates, which presently may be welded simultaneously.
Previously known all-welded plate heat exchangers cannot be taken apart if a leakage arises, instead the whole of the plate heat exchanger must be discarded in case of a possible defect.
As an alternative to an all-welded plate heat exchanger, modules composed of 10-20 heat transfer plates may be welded together. After testing, the modules are assembled to a complete plate heat exchanger by means of intermediate gaskets, which permit the modules to be dismantled from each other and replaced with new modules in case of a possible defect. Such plate heat exchangers are previously known through SE 304 293 and WO 92/11501. The disadvantage with these is that the intermediate gaskets limit the applicability of the plate heat exchangers.
Conventional welding technique normally requires that a welding tool is applied against both plates, at the welding of the two plates. However, modern welding techniques, such as laser-welding and electronic beam welding, facilitates welding of two closely united plates with use of a welding tool only on one side of the two plates. An advantage with modern welding techniques is also that less heat is generated in the plates, which will be welded together.
With help of modern welding techniques it is possible to weld several heat transfer plates together, by gradually piling the plates on each other and successively welding the plates together by applying a welding tool from only one direction. The first plate thus has previously been placed on a horizontal surface, upon which the remaining plates one after another have been located above and been welded firmly to the plate underneath.
A problem is that the material of the heat transfer plates during welding expands locally by the heat arising from the welding. After joining, the material will crimp during the cooling and consequently welding stresses arise after the welding, which leads to deformation of the heat transfer plates. When only two heat transfer plates are welded together into a module the welding stresses act on the neutral plane of the modules, and therefore no actual deformation arises. When a third heat transfer plate is added the new welding stresses will act outside the neutral plane of the module and thereby tend to curve the module. Each additional heat transfer plate curves the module further. Finally, the curvature becomes too large and it will not be possible to straighten the module without collapsing the corrugation pattern of the heat transfer plates.